TABLE 3.
Results of checkerboard synergy testing (FICIs) with non-susceptible and susceptible antimicrobial combinations.
| Antimicrobial | Rangea (mg/L) | MICALONEb (mg/L) | Combination FICIMEAN ± SD (FICILOW)c | |||
| Oxytetracycline | Spectinomycin | Tylosin | Synergy (FICIMEAN ≤0.5) | |||
| Mannheimia haemolytica ATCC 33396 × PM22 transconjugant | ||||||
| Penicillin G | 0.0078–0.5 | 0.125 | 1.11 ± 0.09 (1.00) | 0.59 ± 0.25 (0.25) | 0.65 ± 0.09 (0.56) | No interactions |
| Enrofloxacin | 0.0078–0.5 | 0.250 | 0.64 ± 0.30 (0.31) | 0.70 ± 0.29 (0.38) | 0.57 ± 0.23 (0.31) | No interactions |
| Florfenicol | 0.0625–4 | 1 | 0.68 ± 0.22 (0.50) | 0.61 ± 0.24 (0.38) | 0.81 ± 0.23 (0.56) | No interactions |
| Ceftiofur | 0.0313–2 | 0.125 | 1.28 ± 0.18 (1.06) | 1.28 ± 0.18 (1.06) | 3.29 ± 1.23 (1.50) | No interactions |
| Chlortetracycline | 0.0625–4 | 1 | 1.21 ± 0.18 (1.03) | 1.05 ± 0.51 (0.56) | 0.57 ± 0.24 (0.31) | No interactions |
| Pasteurella multocida CCUG 17976 × PM22 transconjugant | ||||||
| Penicillin G | 0.0078–0.5 | 0.125 | 0.93 ± 0.25 (0.56) | 0.65 ± 0.09 (0.56) | 1.13 ± 0.10 (1.00) | No interactions |
| Enrofloxacin | 0.0078–0.5 | 0.250 | 1.23 ± 0.18 (1.06) | 0.98 ± 0.22 (0.63) | 0.99 ± 0.20 (0.63) | No interactions |
| Florfenicol | 0.0625–4 | 1 | 0.74 ± 0.25 (0.50) | 0.53 ± 0.09 (0.56) | 0.61 ± 0.24 (0.38) | No interactions |
| Ceftiofur | 0.0313–2 | 0.125 | 1.14 ± 0.11 (1.00) | 1.28 ± 0.18 (1.06) | 1.14 ± 0.11 (1.00) | No interactions |
| Chlortetracycline | 0.0625–4 | 1 | 1.13 ± 0.10 (1.00) | 1.13 ± 0.10 (1.00) | 0.61 ± 0.24 (0.38) | No interactions |
aRange of antimicrobial concentrations tested. bMinimum inhibitory concentration (MIC) of antimicrobial tested alone. cFractional inhibitory concentration index (FICI). The FICI values are given as the mean ± standard deviation resulting from three experiments. FICI values in brackets indicate the lowest value for the given combination. Synergism, ≤0.5; No interaction, ≥0.5.